Capacitance detecting device

ABSTRACT

To provide a superior capacitance detecting device, a capacitance detecting device includes M row lines and N column lines that are arranged in a matrix, capacitance detecting elements provided at intersections therebetween, and power lines. The capacitance detecting element includes a signal detecting element and a signal amplifying element. The signal detecting element includes a capacitance detecting electrode, a capacitance detecting dielectric film, and a reference capacitor. The signal amplifying element is composed of a thin film semiconductor device having a gate electrode, a gate insulating film, and a semiconductor film, and an electrode of the reference capacitor is connected to the row line.

BACKGROUND OF THE INVENTION

1. Field of Invention

Exemplary aspects of the present invention relate to a capacitancedetecting device that reads the surface contours of a target havingminute unevenness, such as a fingerprint, by detecting capacitance whichchanges according to a distance from the surface of the target.

2. Description of Related Art

In related art capacitance detecting devices used for, for example,fingerprint sensors, a sensor electrode and a dielectric film providedon the sensor electrode are formed on a monocrystalline siliconsubstrate. See Japanese Unexamined Patent Application Publication No.11-118415 Japanese Unexamined Patent Application Publication Nos.2000-346608, 2001-56204 and 2001-133213. The operating principle of arelated art capacitance detecting device is illustrated in FIG. 10. Thesensor electrode and the dielectric film constitute an electrode and adielectric film of a capacitor, respectively. The other electrode of thecapacitor is a human body connected to the ground. The capacitance C_(F)of this capacitor changes according to the unevenness of a fingerprintcontacting the surface of the dielectric film. The semiconductorsubstrate is equipped with a capacitor that forms a capacitance C_(S).These two capacitors are connected to each other in series, and apredetermined voltage is applied thereto. By application of a voltage,electric charge Q is generated between the two capacitors correspondingto the unevenness of a fingerprint. This electric charge Q is detectedby using ordinary semiconductor technology, whereby the surface contoursof a target are read.

SUMMARY OF THE INVENTION

However, the related art capacitance detecting devices are generallyformed on a monocrystalline silicon substrate. Hence, they can befragile and subject to breaking due to the strong finger pressure whenthey are used as a fingerprint sensor.

Further, fingerprint sensors, because of the application, need to beapproximately 20 mm by 20 mm in size. The majority of the area of thecapacitance detecting device is occupied by the sensor electrodes. Thesensor electrodes are formed on a monocrystalline silicon substrate. Yetthe majority of the monocrystalline silicon substrate (the lowerportions of the sensor electrodes), which is made by using enormousenergy and labor, serves as nothing more than a supporting member. Therelated art capacitance detecting devices are not only expensive, butthey are also unnecessary in view of their limited function.

Furthermore, in recent years, there has been strong demand that cards,such as credit cards and bank cards, having personal identificationfunctions, are secure. However, the related art capacitance detectingdevices formed on a monocrystalline silicon substrate have poorflexibility, and therefore cannot be formed on a plastic substrate.

For this reason, thin film semiconductors may be directly provided on,for example, a plastic substrate. However, since the transistorcharacteristics of the thin film semiconductor formed on the plasticsubstrate are not as good as those of the thin film semiconductor formedon a monocrystalline silicon substrate, it is difficult to accuratelyread a minute variation in electric charge when a fingerprint sensorperforms detection.

Accordingly, exemplary aspects of the present invention are designed toaddress and/or solve the above-mentioned and/or other problems.Exemplary aspects of the present invention provide a capacitancedetecting device capable of sensing capacitance with high accuracy byusing thin film semiconductors. Exemplary aspects of the presentinvention provide a superior capacitance detecting device that operatesstably, can reduce the expenditure of unnecessary energy and laborduring manufacture, and can be made on substrates other than amonocrystalline silicon substrate. Specifically, exemplary aspects ofthe present invention provide a capacitance detecting device that can beexcellently operated by thin film semiconductors.

In order to achieve or address the above, a first exemplary aspect ofthe present invention provides a capacitance detecting device that readssurface contours of a target by detecting capacitance which changesaccording to a distance from the target. The capacitance detectingdevice includes capacitance detecting elements arranged in a matrix of Mrows and N columns and power lines to supply power to the respectivecapacitance detecting elements. Each of the capacitance detectingelements includes a) a signal detecting element to store electric chargecorresponding to capacitance, b) a reset element to reset the electriccharge stored in the signal detecting element, and c) a signalamplifying element to amplify a signal corresponding to the electriccharge stored in the signal detecting element. The signal detectingelement includes a capacitance detecting electrode. The signalamplifying element is composed of a thin film semiconductor device forsignal amplification having a source electrode, a drain electrode, and agate electrode. The reset element is composed of a thin filmsemiconductor device for reset having a source electrode, a drainelectrode, and a gate electrode. The gate electrode of the signalamplifying element, the capacitance detecting electrode, and the drainelectrode of the reset element are connected to each other.

According to the above-mentioned structure of an exemplary aspect of thepresent invention, because the reset element resets the electric chargestored in the signal amplifying element, the amount of the electriccharge stored in the signal detecting element varies according towhether the ridge of a fingerprint or the valley approaches thecapacitance detecting electrode of the signal detecting element, therebyaccurately detecting capacitance. Then, since the signal amplifyingelement amplifies a signal corresponding to the capacitance, it ispossible to accurately detect a minute variation in capacitance evenwhen the capacitance detecting device is formed of thin filmsemiconductor devices.

Herein, the meaning of the term “reset” is to keep electric charge in apredetermined amount, to make the amount of electric charge zero, whichcan be quantitatively grasped, in addition to nearly dischargingelectric charge to zero.

According to a second exemplary aspect of the present invention, acapacitance detecting device that reads surface contours of a target bydetecting capacitance which changes according to a distance from thetarget is formed on a glass substrate using thin film semiconductors.The capacitance detecting device includes M row lines and N column linesarranged in a matrix of M rows and N columns; M×N capacitance detectingelements arranged at the intersections of the row lines and the columnlines; and power lines. Each of the capacitance detecting elementsincludes a signal detecting element; a signal amplifying element; and areset element. The signal detecting element includes at least acapacitance detecting electrode and a capacitance detecting dielectricfilm. The signal amplifying element is composed of a thin filmsemiconductor device for signal amplification having a source electrode,a drain electrode, and a gate electrode. Similarly, the reset element iscomposed of a thin film semiconductor device for reset having a sourceelectrode, a drain electrode, and a gate electrode. A ground potentialis applied to the power lines. The gate electrode of the signalamplifying element, the capacitance detecting electrode, and the drainelectrode of the reset element are connected to each other. In addition,the source electrode of the reset element is connected to the powerline. When the reset element is in a switched-on state, the gateelectrode of the signal amplifying element, the capacitance detectingelectrode, and the power line are electrically connected to each other.Further, the gate electrode of the reset element is connected to acolumn line adjacent to the column line locating the capacitancedetecting element including the reset element.

Furthermore, according to an exemplary aspect of the present invention,when the capacitance detecting element is in a selected state, thesource electrode of the thin film semiconductor device for signalamplification is electrically connected to the power line. Moreover,according to an exemplary aspect of the present invention, thecapacitance detecting device may include output lines, and the drainelectrode of the thin film semiconductor device for signal amplificationis electrically connected to an output line when the capacitancedetecting element is in the selected state. In addition, according to anexemplary aspect of the present invention, the signal amplifying elementand the reset element are thin film semiconductor devices having thesame conductivity type.

A third exemplary aspect of the present invention provides a capacitancedetecting device that reads surface contours of a target by detectingcapacitance which changes according to a distance from the target. Thecapacitance detecting device includes capacitance detecting elementsarranged in a matrix of M rows and N columns; and power lines to supplypower to the respective capacitance detecting elements. Each of thecapacitance detecting elements includes a) a signal detecting element tostore electric charge corresponding to the capacitance, b) a resetelement to reset the electric charge stored in the signal detectingelement, and c) a signal amplifying element to amplify a signalcorresponding to the electric charge stored in the signal detectingelement. The signal detecting element includes a1) a capacitancedetecting electrode, a2) a capacitance detecting dielectric filmprovided on the capacitance detecting electrode, and a3) a referencecapacitor. The reference capacitor has a first electrode, a secondelectrode, and a dielectric film provided between the first electrodeand the second electrode. The signal amplifying element is composed of athin film semiconductor device for signal amplification having a sourceelectrode, a drain electrode, and a gate electrode. The reset element iscomposed of a thin film semiconductor device for reset having a sourceelectrode, a drain electrode, and a gate electrode. The gate electrodeof the signal amplifying element, the capacitance detecting electrode,the second electrode of the reference capacitor, and the drain electrodeof the reset element are connected to each other.

According to the above-mentioned structure of an exemplary aspect of thepresent invention, the reset element resets the electric chargeremaining in the signal detecting element, and the amount of theelectric charge stored in the signal detecting element varies accordingto whether the ridge of a fingerprint or the valley approaches thecapacitance detecting electrode of the signal detecting element, therebyaccurately detecting capacitance. Then, since the signal amplifyingelement amplifies and outputs a signal corresponding to the capacitance,it is possible to accurately detect a minute variation in capacitanceeven when the capacitance detecting device is formed of thin filmsemiconductor devices. In addition, according to an exemplary aspect ofthe present invention, since the reference capacitor is connected to thegate electrode of the signal amplifying element, it is possible tochange the voltage of the gate electrode of the signal amplifyingelement without being affected by the drain voltage or the draincapacitance of the signal amplifying element, and thus to enhance theaccuracy of detection. In addition, it is unnecessary to extremelyincrease a source voltage.

According to a fourth exemplary aspect of the present invention, acapacitance detecting device that reads surface contours of a target bydetecting capacitance which changes according to a distance from thetarget is formed on a glass substrate using thin film semiconductors.The capacitance detecting device includes M row lines and N column linesarranged in a matrix of M rows and N columns; M×N capacitance detectingelements arranged at the intersections of the row lines and the columnlines; and power lines. Each of the capacitance detecting elementsincludes a signal detecting element, a signal amplifying element, and areset element. The signal detecting element includes a capacitancedetecting electrode, a capacitance detecting dielectric film, and areference capacitor. The reference capacitor has a first electrode, adielectric film, and a second electrode. The signal amplifying elementis composed of a thin film semiconductor device for signal amplificationhaving a source electrode, a drain electrode, and a gate, electrode.Similarly, the reset element is composed of a thin film semiconductordevice for reset having a source electrode, a drain electrode, and agate electrode. A ground potential is applied to the power lines. Thegate electrode of the signal amplifying element, the capacitancedetecting electrode, the second electrode of the reference capacitor,and the drain electrode of the reset element are connected to eachother. Further, according to an exemplary aspect of the presentinvention, the first electrode of the reference capacitor and a columnline are electrically connected to each other. When the reset element isin a switched-on state, the gate electrode of the signal amplifyingelement, the capacitance detecting electrode, and the second electrodeof the reference capacitor are electrically connected to the power line.When the reset element in the switched-on state, the first and secondelectrodes of the reference capacitor have the same potential.Furthermore, according to an exemplary aspect of the present invention,the source electrode of the reset element is connected to the powerline. Moreover, according to an exemplary aspect of the presentinvention, the gate electrode of the reset element is connected to acolumn line adjacent to the column line locating the capacitancedetecting element including the reset element. Further, according to anexemplary aspect of the present invention, when the capacitancedetecting element is in a selected state, the source electrode of thethin film semiconductor device for signal amplification is electricallyconnected to the power line. Furthermore, the capacitance detectingdevice of an exemplary aspect of the present invention further includesoutput lines, and the drain electrode of the thin film semiconductordevice for signal amplification is electrically connected to an outputline when the capacitance detecting element is in the selected state.Moreover, according to an exemplary aspect of the present invention, thesignal amplifying element and the reset element are thin filmsemiconductor devices having the same conductivity type.

Further, according to an exemplary aspect of the present invention, acapacitance detecting device includes a signal detecting element, asignal amplifying element, a column selecting element, a row selectingelement, and a reset element. Similar to the above-mentioned aspects,the signal detecting element includes a capacitance detecting electrode,a capacitance detecting dielectric film, and a reference capacitor. Thereference capacitor has a first electrode, a dielectric film, and asecond electrode. The signal amplifying element is composed of a thinfilm semiconductor device for signal amplification having a sourceelectrode, a drain electrode, and a gate electrode. Similarly, thecolumn selecting element is composed of a thin film semiconductor devicefor column selection having a source electrode, a drain electrode, and agate electrode. The row selecting element is composed of a thin filmsemiconductor device for row selection having a source electrode, adrain electrode, and a gate electrode. Also, the reset element iscomposed of a thin film semiconductor device for reset having a sourceelectrode, a drain electrode, and a gate electrode.

Further, according to an exemplary aspect of the present invention, thethin film semiconductor device for signal amplification, the thin filmsemiconductor device for column selection, and the thin filmsemiconductor device for row selection are connected to each other inseries. The gate electrode of the signal amplifying element, thecapacitance detecting electrode, the second electrode of the referencecapacitor, and the drain electrode of the reset element are connected toeach other. Furthermore, according to an exemplary aspect of the presentinvention, the first electrode of the reference capacitor and the columnline are connected to each other. When the reset element in theswitched-on state, the gate electrode of the signal amplifying element,the capacitance detecting electrode, and the second electrode of thereference capacitor are electrically connected to the power line. Inaddition, the first and second electrodes of the reference capacitorhave the same potential when the reset element in the switched-on state.Furthermore, according to an exemplary aspect of the present invention,the source electrode of the reset element is connected to the columnline. Moreover, according to an exemplary aspect of the presentinvention, the gate electrode of the reset element is connected to acolumn line adjacent to the column line locating the capacitancedetecting element including the reset element.

Further, according to an exemplary aspect of the present invention, whenthe capacitance detecting element is in a selected state, the sourceelectrode of the thin film semiconductor device for signal amplificationis electrically connected to the power line. Furthermore, thecapacitance detecting device of an exemplary aspect of the presentinvention may include output lines, and the drain electrode of the thinfilm semiconductor device for signal amplification is electricallyconnected to an output line when the capacitance detecting element is inthe selected state. Moreover, according to an exemplary aspect of thepresent invention, the signal amplifying element and the reset elementare thin film semiconductor devices having the same conductivity type.Further, according to an exemplary aspect of the present invention, thegate electrode of the thin film semiconductor device for columnselection is connected to the column line. Furthermore, the resetelement and the column selecting element are thin film semiconductorhaving the same conductivity type. Moreover, according to an exemplaryaspect of the present invention, when the capacitance detecting deviceincludes the row selecting elements, the gate electrodes of the thinfilm semiconductor devices for row selection are connected to the rowlines.

According to an exemplary aspect of the present invention, thedielectric film of the reference capacitor and the gate insulating filmof the thin film semiconductor device for signal amplification are madeof the same material. These films may be formed on the same layer. Anelectrode of the reference capacitor is made of the same material as adrain region of the thin film semiconductor device for signalamplification. This electrode and the drain region of the thin filmsemiconductor device for signal amplification are formed on the samelayer. Further, the other electrode of the reference capacitor is madeof the same material as the gate electrode of the thin filmsemiconductor device for signal amplification. These electrodes may beformed on the same layer.

Furthermore, according to an exemplary aspect of the present invention,a reference capacitor capacitance CR and a transistor capacitance CT ofthe thin film semiconductor device for signal amplification are definedby the following expressions:C _(R)=∈₀·∈_(R) ·S _(R) /t _(R)C _(T)=∈₀·∈_(ox) ·S _(T) /t _(ox)

-   -   where ∈₀ is a dielectric constant in vacuum, S_(R) (μm²) is the        electrode area of the reference capacitor, t_(R) (μm) is the        thickness of the reference capacitor dielectric film, ∈_(R) is a        relative dielectric constant of the reference capacitor        dielectric film, S_(T) (μm²) is the gate electrode area of the        thin film semiconductor device for signal amplification, t_(ox)        (μm) is the thickness of the gate insulating film, and ∈_(ox) is        a relative dielectric constant of the gate insulating film. In        addition, an element capacitance C_(D) of the signal detecting        element is defined by the following expression:        C _(D)=∈₀·∈_(D) ·S _(D) /t _(D)    -   where ∈₀ is a dielectric constant in vacuum, S_(D) (μm²) is the        area of the capacitance detecting electrode, t_(D) (μm) is the        thickness of the capacitance detecting dielectric film, and        ∈_(D) is a relative dielectric constant of the capacitance        detecting dielectric film. In this case, the element capacitance        C_(D) is sufficiently larger than the sum of the reference        capacitor capacitance C_(R) and the transistor capacitance        C_(T), that is, C_(R)+C_(T). Ideally, the reference capacitor        capacitance C_(R) is sufficiently larger than the transistor        capacitance C_(T). Thus, the element capacitance C_(D) is        sufficiently larger than the reference capacitor capacitance        C_(R). Further, according to an exemplary aspect of the present        invention, the capacitance detecting dielectric film is        positioned on an uppermost surface of the capacitance detecting        device. Furthermore, the target to be measured is not brought        into contact with the capacitance detecting dielectric film, but        is separated therefrom by a target distance t_(A). When a target        capacitance C_(A) is defined by the following equation:        C _(A)=∈₀·∈_(A) ·S _(D) /t _(A)    -   where ∈₀ is a dielectric constant in vacuum, ∈_(A) is a relative        dielectric constant of air, and S_(D) is the area of the        capacitance detecting electrode, the reference capacitor        capacitance C_(R) is sufficiently larger than the target        capacitance C_(A). In this case, ideally, the reference        capacitor capacitance C_(R) is sufficiently larger than the        transistor capacitance C_(T).

Further, an exemplary aspect of the present invention is characterizedin that the capacitance detecting dielectric film is positioned on theuppermost surface of the capacitance detecting device; that, when areference capacitor capacitance C_(R) and a transistor capacitance C_(T)of the thin film semiconductor device for signal amplification aredefined by the following expressions:C _(R)=∈₀·∈_(R) ·S _(R) /t _(R)C _(T)=∈₀·∈_(ox) ·S _(T) /t _(ox)where ∈₀ is a dielectric constant in vacuum, S_(R) (μm²) is theelectrode area of the reference capacitor, t_(R) (μm) is the thicknessof the reference capacitor dielectric film, ∈_(R) is a relativedielectric constant of the reference capacitor dielectric film, S_(T)(μm²) is the gate electrode area of the thin film semiconductor devicefor signal amplification, t_(ox) (μm) is the thickness of the gateinsulating film, and ∈_(ox) is a relative dielectric constant of thegate insulating film, and when an element capacitance C_(D) of thesignal detecting element is defined by the following expression:C _(D)=∈₀·∈_(D) ·S _(D) /t _(D)

-   -   where ∈₀ is a dielectric constant in vacuum, S_(D) (μm²) is the        area of the capacitance detecting electrode, t_(D) (μm) is the        thickness of the capacitance detecting dielectric film, and        ∈_(D) is a relative dielectric constant of the capacitance        detecting dielectric film, then the element capacitance C_(D) is        sufficiently larger than the sum of the reference capacitor        capacitance C_(R) and the transistor capacitance C_(T), that is,        C_(R)+C_(T); and that, when a target capacitance C_(A) is        defined by the following expression:        C _(A)=∈₀·∈_(A) ·S _(D) /t _(A)    -   where ∈₀ is a dielectric constant in vacuum, ∈_(A) is a relative        dielectric constant of air, t_(A) is a distance between the        target and the capacitance detecting dielectric film, and S_(D)        is the area of the capacitance detecting electrode, then the        reference capacitor capacitance C_(R) is sufficiently larger        than the target capacitance C_(A). In this case, ideally, the        reference capacitor capacitance C_(R) is sufficiently larger        than the transistor capacitance C_(T). Thus, the element        capacitance C_(D) is sufficiently larger than the reference        capacitor capacitance C_(R), and the reference capacitor        capacitance C_(R) is sufficiently larger than the target        capacitance C_(A).        [Advantages]

As described above, according to an exemplary aspect of the presentinvention, since a capacitance detecting device includes signalamplifying elements to amplify a signal corresponding to capacitance andreset elements to reset the capacitance, it is possible to sensecapacitance with high accuracy by using thin film semiconductor devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustrating the operating principle of anexemplary aspect of the present invention;

FIG. 2 is a schematic illustrating the circuit structure of acapacitance detecting element according to a first exemplary embodimentof the present invention;

FIG. 3 is a schematic illustrating the principle of the first exemplaryembodiment of the present invention;

FIG. 4 is a second schematic illustrating the principle of the firstexemplary embodiment of the present invention;

FIG. 5 is a schematic illustrating the structure of an element accordingto an exemplary embodiment of the present invention;

FIG. 6 is a schematic illustrating the circuit structure of acapacitance detecting element according to a second exemplary embodimentof the present invention;

FIG. 7 is a schematic illustrating the principle of the second exemplaryembodiment of the present invention (a reset period);

FIG. 8 is a schematic illustrating the principle of the second exemplaryembodiment of the present invention (a reading period);

FIG. 9 is a timing chart for column selection in an exemplary embodimentof the present invention; and

FIG. 10 is a schematic illustrating the operating principle of therelated art.

DETAIL DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, exemplary embodiments of the present invention will bedescribed with reference to the accompanying drawings.

First Exemplary Embodiment

A first exemplary embodiment of the present invention provides acapacitance detecting device that reads out the surface contours of atarget by detecting capacitance which changes according to the distancefrom the target. The capacitance detecting device is manufactured usingthin film semiconductor devices composed of a metal layer, an insulatinglayer, and a semiconductor layer.

Since the thin film semiconductor devices are ordinarily formed on glasssubstrates, semiconductor integrated circuits requiring a large area canbe inexpensively fabricated by using the thin film semiconductordevices. Specifically, they have been recently applied to, for example,liquid crystal display devices. Therefore, when a capacitance detectingdevice applied to a fingerprint sensor is fabricated by using the thinfilm semiconductor devices, it is not necessary to use an expensivesubstrate, such as a monocrystalline silicon substrate, whose productionconsumes tremendous energy. The device can be fabricated inexpensivelywithout wasting precious global resources. A semiconductor integratedcircuit made up of thin film semiconductor devices can be fabricated ona plastic substrate by applying a transfer technique called SUFTLA(which is disclosed in Japanese Unexamined Patent ApplicationPublication No. 11-312811 and S. Utsunomiya et. al., “Society forInformation Display”, pp. 916 (2000)). Therefore, the capacitancedetecting device does not have to be formed on a monocrystalline siliconsubstrate, but can be formed on a plastic substrate.

As shown in FIG. 10, it is impossible to fabricate a capacitancedetection device to which a related art operating principle is appliedusing thin film semiconductor devices with the current technology offabricating thin film semiconductor devices. Since electric charge Qthat is induced between two capacitors connected to each other in seriesis extremely small, the electric charge Q can be accurately read ifmonocrystalline silicon LSI technology, which enables high-accuracydetection, is used. However, the electric charge Q cannot be accuratelyread with a thin film semiconductor device. This is because thetransistor characteristics in a thin film semiconductor device are notas good as the transistor characteristics obtained with themonocrystalline silicon LSI technology, and because there is a largedegree of deviation in characteristics between thin film semiconductordevices.

Further, the capacitance detecting device of the present exemplaryembodiment includes M row lines (M is an integer of 1 or more) and Ncolumn lines (N is an integer of 1 or more) arranged in a matrix of Mrows and N columns. M×N capacitance detecting elements provided at theintersections of the respective row lines and the respective columnlines, and power lines. The capacitance detecting elements each includesa signal detecting element, a signal amplifying element, and a resetelement. The signal detecting element includes at least a capacitancedetecting electrode and a capacitance detecting dielectric film.

As described later, the signal detecting element may include a referencecapacitor in order to increase detection sensitivity at a low voltage.When the signal detecting element includes the reference capacitor, thereference capacitor includes a first electrode, a dielectric film, and asecond electrode.

When a target, such as a finger, approaches or comes into contact withthe capacitance detecting dielectric film, a potential V_(G) isgenerated corresponding to the capacitance between the target and thecapacitance detecting electrode. In the present exemplary embodiment,the potential V_(G) is amplified and inverted into a current or voltageby the signal amplifying element provided in each capacitance detectingelement. Specifically, the signal amplifying element includes a gateelectrode, a gate insulating film, and a semiconductor film, and iscomposed of a thin film semiconductor device for signal amplificationhaving a source electrode, a drain electrode, and a gate electrode.Similarly, the reset element includes a gate electrode, a gateinsulating film, and a semiconductor film, and is composed of a thinfilm semiconductor device for reset having a source electrode, a drainelectrode, and a gate electrode. The gate electrode of the signaldetecting element, the capacitance detecting electrode, and the drainelectrode of the reset element are connected to each other.

Further, when the signal detecting element includes the referencecapacitor, an electrode of the reference capacitor is connected to acolumn line, and the other electrode thereof is connected to thecapacitance detecting electrode, the gate electrode of the thin filmsemiconductor device for signal amplification, and the drain electrodeof the reset element. For example, when the column line and the firstelectrode of the reference capacitor are electrically connected to eachother, the second electrode of the reference capacitor is electricallyconnected to the capacitance detecting electrode, the gate electrode ofthe thin film semiconductor device for signal amplification, and thedrain electrode of the reset element. When the second electrode of thereference capacitor and the column line are electrically connected toeach other, the first electrode of the reference capacitor iselectrically connected to the capacitance detecting electrode, the gateelectrode of the thin film semiconductor device for signalamplification, and the drain electrode of the reset element.

The description of the present invention does not distinguish a sourceelectrode from a drain electrode of a thin film semiconductor device forthe sake of convenience. Herein, an electrode of a thin filmsemiconductor device is referred to as a source electrode, and anotherelectrode is referred to as a drain electrode. Strictly speaking in aphysical conception, in an N-type transistor, an electrode having alower potential is defined as a source electrode. On the other side, ina P-type transistor, an electrode having a higher voltage is defined asa source electrode. Which electrode has a higher potential depends onthe operating state of a transistor. Therefore, strictly speaking, thepositions of a source electrode and a drain electrode are alwayschangeable in a transistor. The description of the present inventionexcludes such strictness for the purpose of the clarity of explanation,and thus an electrode is referred to as a source electrode, and anotherelectrode is referred to as a drain electrode for the sake ofconvenience.

First, the basic operating principles of an exemplary aspect of thepresent invention having the above-mentioned structure will be describedwith reference to FIG. 1. A potential V_(G) induced between a capacitorhaving capacitance C_(F) that varies according to the surface contoursof a target and resultant capacitance C_(R)+C_(T) of a referencecapacitor having capacitance C_(R) and a thin film semiconductor devicefor signal amplification having capacitance C_(T) is connected to a gateelectrode (which is indicated by reference numeral “G” in FIG. 1) of thethin film semiconductor device for signal amplification to change thegate potential of the semiconductor device. When a predetermined voltageis applied to a drain region (which is indicated by alphabet “D” inFIG. 1) of the thin film semiconductor device, a current I_(ds) flowingbetween the source and drain of the thin film semiconductor device ismodulated corresponding to the induced gate potential V_(G). Theelectric charge Q corresponding to the potential V_(G) is stored in thegate electrode. But the electric charge is holed without flowinganywhere, whereby the current value I_(ds) is kept uniform. Therefore, ahigh drain voltage or a long measuring time can be obtained, and thus itis possible to easily measure the current value I_(ds) and to moreaccurately detect the surface contours of a target using the thin filmsemiconductor devices. The signal (a current or voltage) obtained byamplifying information on the capacitance of a target is read outthrough an output line.

In order to measure the capacitance of a target, the current I_(ds)passing through the signal amplifying element may be measured, or avoltage V corresponding to the current I_(ds) passing through the signalamplifying element may be measured. When the reference capacitor is notprovided, in the above-mentioned construction, the capacitance C_(R) isset to zero, and the capacitance of a target is measured using thetransistor capacitance C_(T) and the capacitance C_(F) varying accordingto the surface contours of a target in the same principle as describedabove. Hereinafter, an example in which a reference capacitor isprovided will be described below as an exemplary embodiment of thepresent invention. It is also effective to allow the reference capacitorto function as a transistor capacitance of the signal amplifyingelement.

Next, the circuit structure of the capacitance detecting elementaccording to an exemplary aspect of the present invention will bedescribed with reference to FIG. 2. As described above, each capacitancedetecting element 1 includes a signal amplifying element T2 and a signaldetecting element (4 or 5), which are indispensable components, and areset element T1. The signal detecting element (4 or 5) has at least acapacitance detecting electrode 41 and a capacitance detectingdielectric film 42, and may further include a reference capacitor 5. Thereference capacitor 5 includes a first electrode 51, a dielectric film52, and a second electrode 53. The gate electrode of the signalamplifying element T2, the capacitance detecting electrode 41, anelectrode (the second electrode 53) of the reference capacitor 5, andthe drain electrode of the reset element T1 are connected to each other.Such a structure enables the gate electrode of the signal amplifyingelement T2, the second electrode 53 of the reference capacitor, and thecapacitance detecting electrode 41 to be set to ground potential whenthe reset element T1 is selected to be a switched-on state.Specifically, elements are arranged such that the drain electrode of thereset element T1 has a ground potential when a power line P to which theground potential is applied is connected to the source electrode of thereset element T1, and a reset selection signal is input to the gateelectrode of the reset element T1 to allow the reset element T1 to be aswitched-on state. At the same time, the reference capacitor 5 is alsoarranged such that its first electrode 51 has the ground potential. Inthis way, the potentials of the gate electrode of the signal amplifyingelement, the capacitance detecting electrode, and the second electrodeof the reference capacitor fall to the ground potential for a period oftime when the reset selection signal is supplied. At the same time, thereference capacitor is also arranged such that its first electrode hasthe ground potential. By such wiring and arrangement of the elements, itis possible to select the capacitance detecting element 1 to dischargeunnecessary electric charge from the gate electrode of the signalamplifying element T2 and the capacitance detecting electrode 41 beforemeasuring the capacitance of a target, thereby enhancing the accuracy ofdetection.

The signal amplifying element T2 according to the present exemplaryembodiment is arranged between the power line P and an output line O.For example, a source electrode of the thin film semiconductor devicefor signal amplification, which is a signal amplifying element T2, iselectrically connected to the power line P, and a drain thereof iselectrically connected to the output line O.

The term “electrical connection” means a state in which two members areelectrically connected to each other through a switching element. Ofcourse, the drain electrode may be directly connected to the outputline, and the source electrode may be directly connected to the powerline.

The capacitance detecting device according to an exemplary aspect of thepresent invention reads out the surface contours of a target bysequentially selecting the respective capacitance detecting elements 1arranged in a matrix by the column lines C and the row lines R. FIG. 2shows the structure of each capacitance detecting element 1 including acolumn selecting element T3 and a row selecting element T4. With suchstructure, the respective capacitance detecting elements 1 are uniquelyselected to reduce or prevent the information interference between thecapacitance detecting elements 1, thereby realizing high-accuracydetection at high speed. Specifically, the column selecting element T3is composed of a thin film semiconductor device for column selectionhaving a gate electrode, a gate insulating film, and a semiconductorfilm, and the row selecting element T4 is also composed of a thin filmsemiconductor device for row selection having a gate electrode, a gateinsulating film, and a semiconductor film. When the capacitancedetecting element 1 according to an exemplary aspect of the presentinvention includes the column selecting element T3 and the row selectingelement T4, the thin film semiconductor device for signal amplification,which form the signal amplifying element T2, the thin film semiconductordevice for column selection, and the thin film semiconductor device forrow selection are connected to each other in series. The columnselecting element T3 is provided in each of the capacitance detectingelements 1 to uniquely perform column selection, thereby reducing orpreventing the information interference between columns. In addition,the row selecting element T4 is also provided in each of the capacitancedetecting elements 1 to uniquely perform row selection, thereby reducingor preventing the information interference between rows.

Therefore, it is possible to select only one capacitance detectingelement 1 connected to a column line C and a row line R from the M×Ncapacitance detecting elements 1. When the capacitance detecting element1 includes the column selecting element T3 and the row selecting elementT4, the gate electrode of the thin film semiconductor device for columnselection is connected to the column line C, and the gate electrode ofthe thin film semiconductor device for row selection is connected to therow line R. In FIG. 2, an N-type transistor is used for the rowselecting element T4. Therefore, a low potential (V_(ss)) is applied tonon-selected row lines, and a high potential (V_(dd)) is applied to aselected row line R (for example, an i-th row line). Similarly, in FIG.2, an N-type transistor is used for the column selecting element T3.Therefore, the low potential (V_(ss)) is applied to non-selected columnlines C, and the high potential (V_(dd)) is applied to a selected columnline C (for example, an j-th column line).

In such a structure, first, a row line R (for example, an i-th row line)is selected. Then, all the row selecting elements T4 connected to therow line R are simultaneously switched to a transistor-on state. In thisstate, a specific column line C (for example, an j-th column line) isselected. Only when the specific column line C (for example, an j-thcolumn line) is selected from the N column lines C, the high potential(V_(dd)) is applied to the selected column line C (the j-th columnline), so that the electric conductivity of the column selectingelements T3 connected to the column line C (the j-th column line)increases, whereby the column selecting elements T3 are switched to thetransistor-on state. As a result, the electric conductivity between thepower line P and the output line O is determined by the signalamplifying element T2. Since the first electrode 51 of the referencecapacitor 5 is connected to the column line C that has already been inthe selected state, a high potential is applied to the referencecapacitor 5. The potential corresponding to the capacitance of a targetis applied to the gate electrode of the signal amplifying element T2.Only the capacitance detecting element 1 (a capacitance detectingelement located at an intersection of an i-th row and a j-th column)located at an intersection of the row line R (an i-th row) and thecolumn line C (a j-th column) selected in this way is selected from agroup of M×N capacitance detecting elements. Thus it is possible tomeasure the capacitance of a target at that position. Of course, aP-type transistor may be used for the column selecting element T3. Thehigh potential (V_(dd)) may be applied to the gate electrodes of theP-type transistors in a non-selected state, and the low potential(V_(ss)) may be applied thereto in a selected state. In addition, ap-type transistor may be used for the row selecting element T4. The highpotential (V_(dd)) may be applied to the gate electrodes of the P-typetransistors in a non-selected state, and the low potential (V_(ss)) maybe applied thereto in a selected state.

In the present exemplary embodiment, the gate electrode of the resetelement T1 is connected to a column line C adjacent to the column line Cpositioning the capacitance detecting element 1 including the resetelement T1. That is, a drain electrode of a reset element T1 in acapacitance detecting element 1 located in a j-th column is connected toa capacitance detecting electrode 41, a gate electrode of a signalamplifying element T2 that are located in the j-th column, and a gateelectrode of the reset element T1 is connected to a (j+1)-th column line(a rear column line of the j-th column line) or a (j−1)-th column line C(a front column line of the j-th column line) adjacent to the j-thcolumn line. In this way, a column selection signal to be supplied tothe capacitance detecting element 1 through the (j+1)-th column line or(j−1)-th column line can be used as the reset selection signal relatingto the reset element T1 of the capacitance detecting element 1 locatedin the j-th column. Of course, the effects obtained when the gateelectrode of the reset element T1 is connected to another column line Cother than the column line positioning the capacitance detecting element1 may be the same as in the case in which the gate electrode is notconnected to an adjacent column line C. However, by connecting the gateelectrode of the reset element T1 to the adjacent column line, it ispossible to remove a surplus wiring line and thus to reduce parasiticcapacitance. In addition, as described above, the reset elementaccording to the present exemplary embodiment is to dischargeunnecessary electric charge from the gate electrode of the signalamplifying element T2 and the capacitance detecting electrode 41 beforeselecting a capacitance detecting element 1 to measure the capacitanceof a target. Therefore, the reset element T1 may become a switched-onstate immediately before the capacitance detecting element 1 includingthe reset element T1 is selected. For example, ideally, when the columnselection is performed from a low-numbered column line (that is, thecolumn selection is performed in the order of a (j−1)-th column line, aj-th column line, and a (j+1)-th column line), a gate electrode of areset element T1 in a capacitance detecting element 1 belonging to thej-th column is connected to the (j−1)-th column line C. On the contrary,when the column selection is performed from a high-numbered column line(that is, the column selection is performed in the order of the (j+1)-thcolumn line, the j-th column line, and the (j−1)-th column line), thegate electrode of the reset element T1 in the capacitance detectingelement 1 belonging to the j-th column is connected to the (j+1)-thcolumn line C. When the column selection is performed from thehigh-numbered column line (that is, the column selection is performed inthe order of the (j+1)-th column line, the j-th column line, and the(j−1)-th column line), and when the gate electrode of the reset elementT1 of the capacitance detecting element 1 belonging to the j-th columnis connected to the (j+1)-th column line C, which is a rear column lineof the j-th column line, reset is performed on a j-th column while thecapacitance detecting elements 1 located at the next column (the(j+1)-th column) are operating. In FIG. 2, since an N-type transistor isused for the rest element T1, a circuit is constructed such that the lowpotential (V_(ss)) is applied to the column lines C in a non-selectedstate and the high potential (V_(dd)) is applied thereto only in aselected state. Therefore, when the (j+1)-th column line adjacent to thej-th column line is selected, the reset element T1 of the capacitancedetecting element 1 located in the j-th column becomes a switched-onstate. Since the reset element T1 and the column selecting element T3perform an on or off switching operation by the same signal, it ispreferable that the reset element T1 and the column selecting element T3be transistors having the same conductivity type, from the viewpoint ofa simple circuit structure. For example, when the reset element T1 is anN-type transistor, the column selecting element T3 is also an N-typetransistor. In this way, it is possible to utilize the reset signal tobe supplied to the reset element T1 of the current column line as theselection signal to be supplied to an adjacent column line, withoutusing an element such as an inverter. In addition, when a P-typetransistor is used for the reset element T1, the high potential (V_(dd))is applied to the column line C in a non-selected state, and the lowpotential (V_(ss)) is applied thereto only in a selected state.Therefore, even in this case, the same effects can be obtained. Inaddition, a P-type transistor is used for the column selecting elementT3.

According to the present exemplary embodiment, transistors having thesame conductivity type are used for the reset element T1 and the signalamplifying element T2. As described later, when a positive potential issupplied as a source voltage, that is, when a ground potential of thelow potential (V_(ss)) and the high potential (V_(dd)) are used, thepotential of the column line C varies between the ground potential(V_(ss)) and the high potential (V_(dd)). As shown in FIG. 2, it isconsidered that the first electrode 51 of the reference capacitor 5 isconnected to the column line C, and that the capacitance detectingdielectric film 42 is directly connected to a target, which is theground potential, or is connected thereto through the air. Therefore,the potentials of the second electrode 53 of the reference capacitor 5,the capacitance detecting electrode 41, and the gate electrode of thesignal amplifying element T2 also vary between the ground potential(V_(ss)) and the high potential (V_(dd)). In this case, since the sourceelectrode of the signal amplifying element T2 is electrically connectedto the power line P having the ground potential, the signal amplifyingelement T2 may be formed of an N-type transistor in which a sufficienton-current is received even when the potentials of the source and drainare close to the ground potential. Similarly, the source electrode ofthe reset element T1 is also connected to the power line P, and thepotential of the column line C varies between the ground potential(V_(ss)) and the high potential (V_(dd)). Therefore, the reset elementT1 may be an N-type transistor. When a negative potential is used as thesource voltage, that is, when the ground potential which is the highpotential (V_(dd)) and the low potential (V_(ss)) are used, the sourceelectrode of the signal amplifying element T2 is electrically connectedto the ground potential, so that the potential of the source electrodeof the signal amplifying element T2 varies between the low potential(V_(ss)) and the ground potential (V_(dd)). In addition, the sourceelectrode of the reset element T1 is connected to the power line Phaving the ground potential, and the potential of the column line C towhich the gate electrode is connected varies between the low potential(V_(ss)) and the ground potential (V_(dd)). Therefore, in this case, thesignal amplifying element T2 and the reset element T1 may be P-typetransistors.

In the present exemplary embodiment, the power lines P and the outputlines have various forms since the respective capacitance detectingelements 1 are selected one by one. The power lines P provided in thecapacitance detecting device may have the same number of lines as the Ncolumn lines C and be arranged in the column direction. Also, the powerlines P may have the same number of lines as the M row lines R and bearranged in the row direction. In addition, one power line may beprovided for every two column lines, or may be provided for every tworow lines. Similarly, the output lines O provided in the capacitancedetecting device may have the same number of lines as the N column linesC and extend in the column direction. Also, the output lines O may havethe same number of lines as the M row lines R and extend in the rowdirection. In addition, one output line may be provided for every twocolumn lines, or may be provided for every two row lines. In FIG. 2, thenumber of power lines P is equal to the number of row lines R, that is,M, and the power lines P are arranged in the row direction. In addition,the number of output lines O is equal to the number of column lines C,that is, N, and the output lines O extend in the column direction.

When each capacitance detecting element 1 includes the column selectingelement T3 and the row selecting element T4, there is an advantage inthat only a specific capacitance detecting element will be reliablyselected from a group of M×N capacitance detecting elements as describedabove. If the reference capacitor 5 is not provided in the capacitancedetecting element, the transistor capacitance of the signal amplifyingelement T2 is capacitively coupled with the capacitance of a target, andthe product of the capacitance ratio thereof and the drain voltage isapplied to the gate of the signal amplifying element T2. However, sincethe column selecting element T3, the row selecting element T4, and thesignal amplifying element T2 are connected to each other in series, thedrain potential of the signal amplifying element T2 drops from the highpotential (V_(dd)) applied to the output line O by the potentialcorresponding to the presence of the column selecting element T3 and therow selecting element T4. For example, assuming that the electricconductivities of the column selecting element T3, the row selectingelement T4, and the signal amplifying element T2 are equal to each otherin their on states, the drain potential of the signal amplifying elementT2 when the potential (V_(dd)) is applied to the output line O drops toabout one third of the potential (V_(dd)), that is, V_(dd)/3. Therefore,even though the capacitance of a target to be measured varies, avariation in the gate potential of the signal amplifying element T2decreases by a maximum of V_(dd)/3, and then the accuracy of detectiondecreases. Thus, the increase of the value of V_(dd) is required. In thepresent exemplary embodiment, in order to solve and/or address the aboveand/or other problems, the reference capacitor 5 is further provided,and the first electrode 51 of the reference capacitor 5 is directlyconnected to the column line C. Then, for example, even when the columnselecting element T4 or the row selecting element T3 is provided, thegate potential of the signal amplifying element T2 is in a potentialrange of the minimum V_(ss) to the maximum V_(dd) since the highpotential (V_(dd)) is surely applied to the first electrode 51 of thereference capacitor 5. That is, according to the structure of thepresent exemplary embodiment, for example, even when the columnselecting element T4 and the row selecting element T3 are connected tothe signal amplifying element T2 in series between the power line P andthe output line O, the gate potential of the signal amplifying elementT2 can vary in the range of a negative source potential (V_(ss): theground potential) to a positive source potential (V_(dd): the highpotential) according to the capacitance of a target. When the gatepotential of the signal amplifying element T2 is about the negativesource potential, the thin film semiconductor device for signalamplification turns to an off state, so that the electric conductivityof the signal amplifying element T2 is reduced. When the gate potentialof the signal amplifying element T2 is about the positive sourcepotential, the thin film semiconductor device for signal amplificationturns to an on state, so that the electric conductivity of the signalamplifying element T2 increases. Therefore, by measuring the variationof electric conductivity through the output line as described above, itis possible to obtain the unevenness information on the surface of atarget.

The first electrode 51 of the reference capacitor 5 is connected to thecolumn line C, and the second electrode 53 thereof is connected to thecapacitance detecting electrode 41 and the gate electrode of the signalamplifying element T2, which is the thin film semiconductor device forsignal amplification. As described above, in FIG. 2, since the highpotential is applied to the column line C in a state in which the columnline C is selected, the high potential (V_(dd)) is also applied to thefirst electrode 51 of the reference capacitor 5 connected to the columnline C, and the potential corresponding to the capacitance of a targetis applied to the gate electrode of the signal amplifying element T2.Then, the electric conductivity between the source and drain of the thinfilm semiconductor device for signal amplification varies, and thevariation is detected, thereby obtaining the unevenness information onthe surface of a target, for example, fingerprint information.

In the above-mentioned structure, in order for the thin filmsemiconductor device (T2) for signal amplification of the presentexemplary embodiment to effectively amplify signals, the transistorcapacitance C_(T) of the thin film semiconductor device for signalamplification, the reference capacitor capacitance C_(R), and theelement capacitance C_(D) of the signal detecting element (4 or 5) mustbe appropriately set. The relationship therebetween will be explainedbelow with reference to FIGS. 3 and 4.

First, a situation will be considered in which a convex portion of atarget to be measured touches the capacitance detecting dielectric film42, and the target is electrically connected to the ground.Specifically, a situation is assumed in which a capacitance detectingdevice is used as a fingerprint sensor, and the ridges of a fingerprintthat is in contact with the surface of the capacitance detecting deviceare detected. The reference capacitor capacitance C_(R) and thetransistor capacitance C_(T) of the thin film semiconductor device (T2)for signal amplification are respectively defined by the followingexpressions:C _(R)=∈₀·∈_(R) ·S _(R) /t _(R)C _(T)=∈₀·∈_(ox) ·S _(T) /t _(ox)

-   -   where ∈₀ is a dielectric constant in vacuum, S_(R) (μm²) is an        electrode area of the reference capacitor 5, t_(R) (μm) is a        thickness of the reference capacitor dielectric film 52, ∈_(R)        is a relative dielectric constant of the reference capacitor        dielectric film 52, S_(T) (μm²) is an area of the gate electrode        of the thin film semiconductor device (T2) for signal        amplification, t_(ox) (μm) is a thickness of the gate insulating        film, and ∈_(ox) is a relative dielectric constant of the gate        insulating film. Moreover, the element capacitance C_(D) of the        signal detecting element (4 or 5) is defined by the following        expression:        C _(D)=∈₀·∈_(D) ·S _(D) /t _(D)    -   where ∈₀ is a dielectric constant in vacuum, S_(D) (μm²) is the        area of the capacitance detecting electrode 41, t_(D) (μm) is        the thickness of the capacitance detecting dielectric film 42,        and ∈_(D) is a relative dielectric constant of the capacitance        detecting dielectric film 42. The surface of the target serves        as the ground electrode of the element capacitance C_(D), and        the capacitance detecting electrode 41 corresponds to the other        electrode. The capacitance detecting dielectric film 42 is        interposed between the two electrodes. Since the capacitance        detecting electrode 41 is connected to the gate electrode of the        thin film semiconductor device (T2) for signal amplification and        the second electrode 53 of the reference capacitor 5, the        capacitor having the element capacitance C_(D) and the capacitor        having the transistor capacitance C_(T) are connected to each        other in series. At the same time, the capacitor having the        element capacitance C_(D) is also connected in series to the        capacitor having the reference capacitor capacitance C_(R). The        first electrode 51 of the reference capacitor 5 is connected to        the column line C, and the high potential (V_(dd)) is applied        when the column line C is selected. When a positive power source        is used as a source voltage, specifically, when the ground        potential is supplied to the power line P such that the output        line O has the high potential (V_(dd)), the signal amplifying        element T2 is connected in series to the column selecting        element T3 and the row selecting element T4 between the power        line P and the output line O. Therefore, the gate potential of        the thin film semiconductor device for signal amplification when        the column line C is selected is k times the potential V_(dd)        (0<k≦1) (in FIG. 3). The value of k is determined by a        resistance value of the column selecting element T3, a        resistance value of the row selecting element T4, and a        resistance value of the signal amplifying element T2.        Specifically, the value of k is more than zero and 1 or less.        When neither the column selecting element T3 nor the row        selecting element T4 is provided, the value of k is 1. Since the        voltage applied to the column line C and the drain electrode of        the signal amplifying element T2 are divided corresponding to        the capacitances of the three capacitors, the voltage (the gate        voltage when a convex portion is brought into contact therewith)        V_(GT) applied to the gate electrode of the thin film        semiconductor device (T2) for signal amplification in this state        is as follows:

$\begin{matrix}{V_{GT} = {\frac{{k\; C_{T}} + C_{R}}{C_{D} + C_{T} + C_{R}} \cdot V_{dd}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$Therefore, when the element capacitance C_(D) is sufficiently largerthan the sum of the reference capacitor capacitance C_(R) and thetransistor capacitance C_(T), that is, C_(R)+C_(T), as in Equation 2,C _(D) >>C _(T) +C _(R)  Equation 2the gate voltage V_(GT) approximates to zero as in Equation 3, and thusalmost no voltage is applied to the gate electrode.V_(GT)≈0  Equation 3As a result, the thin film semiconductor device for signal amplificationturns to the off state, so that a current I is extremely small.Ultimately, in order that almost no current passes through the signalamplifying element when the convex portion of a target, corresponding tothe ridge of a fingerprint, contacts the capacitance detecting device,the area of the gate electrode (the length or width of the gateelectrode), the material of the gate insulating film, the thickness ofthe gate insulating film, the electrode area of the reference capacitor(the length or width of the capacitor electrode), the material of thereference capacitor dielectric film, the thickness of the referencecapacitor dielectric film, the area of the capacitance detectingelectrode, the material of the capacitance detecting dielectric film,and the thickness of the capacitance detecting dielectric film thatconstitute the capacitance detecting element must be appropriately setsuch that the element capacitance C_(D) is sufficiently larger than thesum of the reference capacitor capacitance C_(R) and the transistorcapacitance C_(T), that is, C_(R)+C_(T). In general, the term“sufficiently larger” means a difference in the magnitude of about 10times or greater. The element capacitance C_(D), the sum of thereference capacitor capacitance C_(R) and the transistor capacitanceC_(T), that is, C_(R)+C_(T), should satisfy the following relationship:C _(D)>10×(C _(R) +C _(T))

In this case, V_(GT)/V_(dd) is approximately 0.1 or less, and the thinfilm semiconductor device cannot turn to the on state. In order toaccurately detect the convex portion of a target, it is important thatthe thin film semiconductor device for signal amplification be in theoff state when the convex portion of the target contacts the capacitancedetecting device. Therefore, if a source voltage is the high potential(V_(dd)), an N-type enhancement mode transistor (a normally off type) inwhich a drain current does not flow at a gate voltage near zero ispreferably used as the thin film semiconductor device for signalamplification. More ideal is to use an N-type MIS thin filmsemiconductor device for signal amplification in which the minimum gatevoltage satisfies the following relationships:0<0.1×V _(dd) <V _(min), or0<V _(GT) <V _(min)

-   -   where V_(min) is a gate voltage (the minimum gate voltage) at        which the drain current becomes the minimum value in transfer        characteristics.

If the source voltage is the low potential (V_(ss)) and the groundpotential is supplied as the high potential (V_(dd)), a P-typeenhancement mode transistor (a normally off type) in which a draincurrent does not flow at a gate voltage near zero is used as the thinfilm semiconductor device for signal amplification. More ideal is to usea P-type MIS thin film semiconductor device for signal amplificationwhose minimum gate voltage V_(min) satisfies the followingrelationships:V _(min)<0.1×V _(ss)<0, orV _(min) <V _(GT)<0These relationships enable the convex portion of the target to beaccurately detected under the situation where the current value I isextremely small.

Next, a situation is considered in which a target does not come intocontact with the capacitance detecting dielectric film, but is separatedtherefrom by a target distance t_(A). A concave portion of the target tobe measured is located above the capacitance detecting dielectric film,and the target is also electrically connected to the ground.Specifically, it is assumed that the valley of a fingerprint approachingthe surface of the capacitance detecting device is detected when thecapacitance detecting device is used as a fingerprint sensor. Asdescribed above, in the capacitance detecting device of an exemplaryaspect of the present invention, the capacitance detecting dielectricfilm may be located on the uppermost surface of the capacitancedetecting device. A schematic of this case is shown in FIG. 4. Since thesurface of the target does not come into contact with the capacitancedetecting dielectric film, an additional capacitor with air as adielectric substance is formed between the capacitance detectingdielectric film and the surface of the target. This is called a targetcapacitance C_(A), and is defined as follows:C _(A)=∈₀·∈_(A) ·S _(D) /t _(A)

-   -   where ∈₀ is a dielectric constant in vacuum, ∈_(A) is a relative        dielectric constant in air, and S_(D) is the area of the        capacitance detecting electrode. As such, in a state in which        the target is separated from the capacitance detecting        dielectric film, the element capacitance C_(D) and the target        capacitance C_(A) are connected to each other in series, and the        transistor capacitance C_(T) and the reference capacitor        capacitance C_(R) connected in parallel to those capacitors are        connected to each other in series. The voltage V_(dd) is applied        to the reference capacitor, and a voltage kV_(dd) is applied to        the drain electrode of the signal amplifying element (FIG. 4).        The applied voltage is divided by the capacitances of the four        capacitors. Therefore, a voltage V_(GV) (the gate voltage when        the valley of a fingerprint approaches) applied to the gate        electrode of the thin film semiconductor device for signal        amplification under such a condition is expressed by the        following Equation 4.

$\begin{matrix}{V_{GV} = {\frac{{k\; C_{T}} + C_{R}}{\frac{C_{A}C_{D}}{C_{A} + C_{D}} + C_{T} + C_{R}} \cdot V_{dd}}} & {{Equation}\mspace{14mu} 4}\end{matrix}$In an exemplary aspect of the present invention, the capacitancedetecting element is formed such that the drain current is extremelysmall when the target comes into contact with the capacitance detectingdevice, so that the condition C_(D)>>C_(T)+C_(R) (Equation 2) issatisfied. Therefore, the voltage V_(GV) approximates to the followingEquation 5:

$\begin{matrix}{V_{GV} \approx \frac{V_{dd}}{\frac{C_{T} + C_{R}}{{k\; C_{T}} + C_{R}} + \frac{C_{A}}{{k\; C_{T}} + C_{R}}}} & {{Equation}\mspace{14mu} 5}\end{matrix}$Herein, if the reference capacitor capacitance C_(R) is sufficientlylarger than the target capacitance C_(A) as in Equation 6,C_(R)>>C_(A)  Equation 6the gate voltage V_(GV) is simply expressed by Equation 7.

$\begin{matrix}{V_{GV} \approx {\frac{{k\; C_{T}} + C_{R}}{C_{T} + C_{R}} \cdot V_{dd}}} & {{Equation}\mspace{14mu} 7}\end{matrix}$As such, when the value of k approximates to 1, the gate voltage V_(GV)is substantially equal to the source voltage V_(dd). When the referencecapacitor capacitance C_(R) is sufficiently larger than the transistorcapacitance C_(T) as in the following Equation 8,C_(R)>>C_(T)  Equation 8the gate voltage V_(GV) is substantially equal to the source voltageV_(dd) regardless of the magnitude of the value of K as in the followingEquation 9.V_(GV)≈V_(dd)  Equation 9As a result, the thin film semiconductor device for signal amplificationcan turn to the on state, and an extremely large current I is obtained.In order for a large amount of current to pass through the signalamplifying element when a concave portion of a target, corresponding tothe valley of a fingerprint, is present over the capacitance detectingdevice, it is necessary that the reference capacitor capacitance C_(R)be sufficiently larger than the target capacitance C_(A). As describedabove, since a difference in magnitude of about 10 times is generallyconsidered “sufficiently larger,” the reference capacitor capacitanceC_(R) and the target capacitance C_(A) may satisfy the followingrelationship:C _(R)>10×C _(A)In order to allow the transistor to be the on state regardless of thevalue of k when the valley of a fingerprint approaches, the referencecapacitor capacitance C_(R) may be ten times or more larger than thetransistor capacitance C_(T) as in the following expression:C _(R)>10×C _(T)When the above-mentioned relationships are satisfied, V_(GV)/V_(dd) isapproximately 0.9 or greater. Thus, the thin film semiconductor deviceeasily turns to the on state. In order to accurately detect the concaveportion of a target, it is important that the thin film semiconductordevice for signal amplification be in the on state when the concaveportion of the target approaches the capacitance detecting device. If apositive power source is used for a source voltage V_(dd), an N-typeenhancement mode transistor (a normally off type) is used as the thinfilm semiconductor device for signal amplification, and a thresholdvoltage V_(th) of the transistor may be smaller than the voltage V_(GV).More ideal is to use an N-type MIS thin film semiconductor device forsignal amplification satisfying the following relationship:0<V _(th)<0.91×V _(dd)On the contrary, if a negative power source is used for the sourcevoltage V_(ss), a P-type enhancement mode transistor (a normally offtype) is used as the thin film semiconductor device for signalamplification. Ideally, the threshold voltage V_(th) of the P-type thinfilm semiconductor device for signal amplification may be larger thanthe voltage V_(GV). More ideal is to use a P-type MIS thin filmsemiconductor device for signal amplification satisfying the followingrelationship:0.91×V _(ss) <V _(th)<0In this way, the concave portions of the target can be accuratelydetected under the situation in which the current value I is extremelylarge.

Ultimately, the correct recognition of the unevenness of the target isperformed in such a way that the signal amplifying element passes almostno current when a convex portion of a target, corresponding to the ridgeof a fingerprint, comes into contact with the capacitance detectingdevice, and that the signal amplifying element passes a large amount ofcurrent when a concave portion of the target, corresponding to thevalley of a fingerprint, is over the capacitance detecting device.Therefore, in order to achieve the correct recognition of the unevennessof the target, it is necessary that the capacitance detecting dielectricfilm in the capacitance detecting element be positioned on the uppermostsurface of the capacitance detecting device, and that the gate electrodearea S_(T) (μm²), the thickness t_(ox) (μm) of the gate insulating film,a relative dielectric constant ∈_(ox) of the gate insulating film of thethin film semiconductor device for signal amplification, the electrodearea S_(R) (μm²) of the reference capacitor, the thickness t_(R) (μm) ofthe reference capacitor dielectric film, a relative dielectric constant∈_(R) of the reference capacitor dielectric film, the area S_(D) (μm²)of the capacitance detecting electrode, the thickness t_(D) (μm) of thecapacitance detecting dielectric film, and a relative dielectricconstant ∈_(D) of the capacitance detecting dielectric film all beappropriately set such that the element capacitance C_(D) issufficiently larger than the sum of the reference capacitor capacitanceC_(R) and the transistor capacitance C_(T), that is, C_(R)+C_(T).Furthermore, it is necessary that the capacitance detecting device beconfigured such that the reference capacitor capacitance C_(R) issufficiently larger than the target capacitance C_(A) when the targetdoes not come into contact with the capacitance detecting dielectricfilm, but is separated therefrom by the target distance t_(A). Ideally,the reference capacitor capacitance C_(R) must be sufficiently largerthan the transistor capacitance C_(T). Specifically, the referencecapacitor capacitance C_(R) and the transistor capacitance C_(T) satisfythe following relationship:C _(R)>10×C _(T)The capacitance detecting device may be configured such the elementcapacitance C_(D), the reference capacitor capacitance C_(R), and thetarget capacitance C_(A) satisfy the following relationships:C _(D)>10×C _(R)C _(R)>10×C _(A)Further, if the high potential (V_(dd)) is used for the source voltage,an N-type enhancement mode transistor (a normally off type) may be usedas the thin film semiconductor device for signal amplification. It isideal to use an N-type enhancement mode transistor whose minimum gatevoltage V_(min) satisfies the following relationship:0<0.1×V _(dd) <V _(min) or 0<V _(GT) <V _(min)and whose threshold voltage V_(th) is smaller than the V_(GV), andspecifically, satisfies the following relationship:0<V _(th)<0.91×V _(dd) or 0<V _(th) <V _(GV)If a negative power source (V_(ss)) is used for the source voltage, aP-type enhancement mode transistor (a normally off type) may be used asthe thin film semiconductor device for signal amplification. It is idealto use a P-type enhancement mode transistor whose minimum gate voltageV_(min) satisfies the following relationship:V _(min)<0.1×V _(ss)<0 or V _(min) <V _(GT)<0and whose threshold voltage V_(th) is larger than the V_(GV), andspecifically, satisfies the following relationship:0.91×V _(ss) <V _(th)<0 or V _(GV) <V _(th)<0

The structure of the capacitance detecting element according to thepresent exemplary embodiment will now be described with reference toFIG. 5. The thin film semiconductor device for signal amplificationconstituting the signal amplifying element T2 of the capacitancedetecting element 1 includes a semiconductor film 110 having a sourceregion, a channel forming region, and a drain region, a gate insulatingfilm 120, and a gate electrode 170 as indispensable components.Similarly, the thin film semiconductor device for column selectionconstituting the column selecting element T3 also includes asemiconductor film 110 having a source region, a channel forming region,and a drain region, a gate insulating film 120, and a gate electrode 170as indispensable components. Further, the thin film semiconductor devicefor row selection constituting the row selecting element T4 includes asemiconductor film 110 having a source region, a channel forming region,and a drain region, a gate insulating film 120, and a gate electrode 170as indispensable components. In the structure shown in FIG. 5, the thinfilm semiconductor device (T2) for signal amplification, the columnselecting element T3, and the row selecting element T4 are respectivelyformed by an NMOS. Although not shown in FIG. 5, the first electrode ofthe reference capacitor is composed of an N-type semiconductor film,which is the same material as the drain region of the thin filmsemiconductor device (T2) for signal amplification. Both the firstelectrode of the reference capacitor and the drain electrode of the thinfilm semiconductor device are formed on the same base protective film.The dielectric film of the reference capacitor is composed of a siliconoxide film, which is the same material as the gate insulating film 120(which is indicated by “G1” in FIG. 5) of the thin film semiconductordevice for signal amplification, and they are formed on the same layer(on the semiconductor layer). The second electrode of the referencecapacitor is composed of a metal film (specifically, a tantalum thinfilm), which is the same material as the gate electrode 170 of the thinfilm semiconductor device for signal amplification (T2).

The capacitance detecting element 1 can be formed on a plastic substrate100 using the above-mentioned SUFTLA technique. A fingerprint sensorbased on a monocrystalline silicon technique has no practical usebecause it vertically splits on plastic or it is not formed in asufficiently large size. However, the capacitance detecting elementformed on the plastic substrate 100 according to an exemplary aspect ofthe present invention can be suitable for a fingerprint sensor on theplastic substrate 100 since there is no fear that the capacitancedetecting element 1 will split even when it has a sufficiently largearea to cover a finger. Specifically, a smart card having a personalcertification function can be realized by the present invention. Thesmart card having the personal identification function is used in cashcards (bank cards), credit cards, and identity cards, and has thesuperior feature of not only markedly enhancing the security level ofthese cards, but also of protecting personal fingerprint informationfrom being released outside the card.

EXAMPLE 1

A capacitance detecting device composed of thin film semiconductordevices is fabricated on a glass substrate. The fabricated capacitancedetecting device is then transferred to a plastic substrate using theSUFTLA technique, thereby forming a capacitance detecting device on aplastic substrate. The capacitance detecting device includes capacitancedetecting elements aligned in a matrix of 304 rows and 304 columns. Thematrix is formed in a square shape having a diagonal measurement of 20mm.

The substrate made of poly-ether-sulfone (PES) has a thickness of 200μm. The signal amplifying element, the row selecting element, the columnselecting element, and the reset element are respectively formed of anN-type thin film semiconductor device. The thin film transistors are atop-gate type as shown in FIG. 5, and are fabricated in alow-temperature process where the maximum process temperature is 425° C.The gate electrode lengths L of the thin film semiconductor device forcolumn selection, and the thin film semiconductor device for rowselection are 3 μm. The gate electrode widths W thereof are 5 μm. Thegate electrode length L of the thin film semiconductor device for signalamplification and the thin film semiconductor device for reset are setto 2 μm, and the gate electrode width W thereof are set to 2 μm. Thesemiconductor film is a polycrystalline silicon thin film achievedthrough laser crystallization and has a thickness of 50 nm. In addition,the gate insulating film is a silicon oxide film having a thickness of45 nm that is formed by a chemical vapor deposition (CVD) method. Thegate electrode is composed of a tantalum thin film having a thickness of400 nm. The relative dielectric constant of the silicon oxide filmconstituting the gate insulating film is found to be approximately 3.9according to CV measurement. The first electrode of the referencecapacitor is formed of an N-type semiconductor film that is the same asthe drain region of the thin film semiconductor device for signalamplification, and the dielectric film of the reference capacitor isformed of a silicon oxide film that is the same as the gate insulatingfilm of the thin film semiconductor device for signal amplification. Thesecond electrode of the reference capacitor is formed of a tantalum thinfilm that is the same as the gate electrode of the thin filmsemiconductor device for signal amplification. The first electrode ofthe reference capacitor is connected to the row line through a contacthole, and the second electrode thereof is connected to the capacitancedetecting electrode and the gate electrode of the N-type thin filmsemiconductor device for signal amplification. The circuit structure ofthe capacitance detecting element is the same as that in FIG. 2. Thegate electrode of the reset element in the capacitance detecting elementpositioned in the j-th column is connected to the (j+1)-th column linethat is selected immediately before the j-th column line, and is commonto the gate electrode of the thin film semiconductor device for columnselection in the capacitance detecting element that is positioned in the(j+1)-th column. The source electrode of the reset element is connectedto the power line having the ground potential, and the drain electrodethereof is connected to the capacitance detecting electrode, the gateelectrode of the thin film semiconductor device for signalamplification, and the second electrode of the reference capacitor.

In this exemplary embodiment, the pitch of the rows and columns thatform the capacitance detecting device is 66 μm, and the resolution is385 dpi (dots per inch). Therefore, the area of the capacitancedetecting electrode is 1529 μm². The capacitance detecting dielectricfilm is formed of a silicon nitride film having a thickness of 300 nm.Since CV measurement shows the relative dielectric constant of thesilicon nitride film to be approximately 7.5, the element capacitanceC_(D) is approximately 338 fF (femtofarad). Assuming that thecapacitance detecting device of the present exemplary embodiment is afingerprint sensor, the target capacitance C_(A) when the valley of afingerprint is present over the surface of the capacitance detectingdevice is calculated to be 0.27 fF since the difference in heightbetween the ridge and valley of a fingerprint is approximately 50 μm.Because the gate electrode length L of the MIS thin film semiconductordevice for signal amplification is set to 2 μm and the gate electrodewidth W thereof is set to 2 μm, the transistor capacitance C_(T) isapproximately 3.07 fF. The electrode area S_(R) of the referencecapacitor is 42 μm². As a result, the reference capacitor capacitanceC_(R) is 32 fF. Thus, the capacitance detecting element described inthis embodiment satisfies the following relationship:C _(D)>10×C _(R)C _(R)>10×C _(T)C _(R)>10×C _(A)

Thus, if the source voltage V_(dd) is 3.3V, the voltage V_(GT) appliedto the gate electrode of the MIS thin film semiconductor device forsignal amplification when the ridge of a fingerprint touches the surfaceof the capacitance detecting device is 0.30 V; and the voltage V_(GV)applied to this gate electrode when the valley of the fingerprint isover the surface is 3.11 V. Since the minimum gate voltage V_(min) ofthe N-type thin film semiconductor device for signal amplification usedin the present exemplary embodiment is 0.35 V, and is larger than 0.30V, which is the gate voltage V_(GT) at the time of the contact of theridge of a fingerprint, the N-type thin film semiconductor device forsignal amplification completely turns to the off state. On the otherside, since the threshold voltage V_(th) is 1.42 V, and is smaller than3.11 V, which is the gate voltage V_(GV) obtained when the valley of thefingerprint approaches, the N-type thin film semiconductor device forsignal amplification completely turns to the on state. As a result, thecurrent value output from the signal amplifying element when the ridgeof a fingerprint contacts the surface of the capacitance detectingdevice is 4.5×10⁻¹³ A, which is extremely small. On the contrary, alarge current of 2.6×10⁻⁵ A is output from the signal amplifying elementwhen the valley of the fingerprint approaches, thereby accuratelydetecting the unevenness information of a fingerprint and the like.

As described above, according to the first exemplary embodiment, it ispossible to manufacture a capacitance detecting element capable ofhigh-accuracy detection by using thin film semiconductor devices.

Particularly, exemplary aspects of the present invention has thefollowing effects by including the reset element.

1) The reset element enables the accurate detection of capacitance whenthe potential of a node G, which is a measurement potential, isuniformly maintained before the read of capacitance.

2) In the first exemplary embodiment, since the power line has theground potential, the reset potential is also the ground potential.Therefore, it is possible to set the voltage according to the variablecapacitance C_(F) of the capacitance detecting element to be zero.Therefore, even when the value of the variable capacitance C_(F) variesdue to the movement of a target before the read of capacitance, thevoltage V_(G) of the node G does not vary, thereby enhancing theaccuracy of detection.

3) In the related art technique using a monocrystalline siliconsubstrate, a small capacitance detecting device having a size of severalmillimeters by several millimeters is formed on a plastic substrate.However, according to the present exemplary embodiment having theabove-mentioned structure, a capacitance detecting device having an areaabout 100 times larger than the related art capacitance detecting devicecan be formed on a plastic substrate, and it is possible to detect theridge and valley information of a target with higher accuracy. As aresult, the present invention can be used, for example, to markedlyenhance the security level of a smart card. In addition, the related artcapacitance detecting device formed on a monocrystalline siliconsubstrate wastes a tremendous amount of energy and labor because only anextremely small portion of the device area actually uses themonocrystalline silicon semiconductor. In contrast to this, the presentinvention eliminates this kind of extravagant waste and has the effectof helping to conserve the global environment.

Second Exemplary Embodiment

The second exemplary embodiment of the present invention relates to anoperation in which a predetermined potential, not the ground potential,is applied to the power line.

As described in the first exemplary embodiment, in the capacitancedetecting element of an exemplary aspect of the present invention, thevariation of capacitance between the capacitance detecting electrode 41and a target is regarded as the variation of the voltage V_(G) of thenode to which the gate electrode of the signal amplifying element isconnected, and the signal amplifying element amplifies the voltageV_(G). Since the voltage between the gate and source of the signalamplifying element T2 does not depend on a supply voltage from the powerline, the effects 1) and 3) of the first exemplary embodiment can beachieved without setting the supply potential of the power line to bethe ground potential.

The circuit structure of a capacitance detecting device according to thesecond exemplary embodiment will be described with reference to FIG. 6.The circuit structure of the capacitance detecting device according tothe second exemplary embodiment is completely the same as that in thefirst exemplary embodiment, except that the potential applied to thepower line P is a predetennined potential V_(PL), not the groundpotential. That is, the capacitance detecting elements 1 are arranged ina matrix of M rows and N columns, and the power lines P are provided forsupplying power to the respective capacitance detecting elements 1. Apredetermined potential V_(PL) is applied to the power line P. Similarto the first exemplary embodiment, the capacitance detecting elements 1each include a signal detecting element (4 or 5) to store electriccharge corresponding to the capacitance, a reset element T1 to reset theelectric charge stored in the signal detecting element, and a signalamplifying element T2 to amplify the signal corresponding to theelectric charge stored in the signal detecting element. The signaldetecting element includes a capacitance detecting electrode 41, acapacitance detecting dielectric film 42 provided on the capacitancedetecting electrode 41, and a reference capacitor 5. The referencecapacitor 5 has a first electrode 51, a second electrode 53, and adielectric film 52 provided between the first electrode 51 and thesecond electrode 53. The signal amplifying element T2 is a thin filmsemiconductor device for signal amplification having a source electrode,a drain electrode, and a gate electrode, and the reset element T1 is athin film semiconductor device for reset having a source electrode, adrain electrode, and a gate electrode. The gate electrode of the signalamplifying element T2, the capacitance detecting electrode 41, thesecond electrode 53 of the reference capacitance, and the drainelectrode of the reset element T1 are connected to each other, and thiscontact point is the node G, the potential of which is a measurementpotential.

In the second exemplary embodiment, the signal amplifying element T2 isarranged between the power line P and the output line O, similar to thefirst exemplary embodiment. Specifically, the source electrode of thethin film semiconductor device for signal amplification, which is thesignal amplifying element T2, is electrically connected to the powerline P, and the drain electrode thereof is electrically connected to theoutput line O through the column selecting element T3 and the rowselecting element T4. The meaning of the term “electrical connection” isthe same as that in the first exemplary embodiment. The column selectingelement T3 is composed of a thin film semiconductor device for columnselection having a gate electrode, a gate insulating film, and asemiconductor film, and the row selecting element T4 is also composed ofa thin film semiconductor device for row selection having a gateelectrode, a gate insulating film, and a semiconductor film. The signalamplifying element T2, the column selecting element T3, and the rowselecting element T4 are connected to each other in series. The gateelectrode of the column selecting element T3 is connected to a columnline C, and the gate electrode of the row selecting element T4 isconnected to a row line R. In the second exemplary embodiment, since thecolumn selecting element T3 and the row selecting element T4 arerespectively formed of an N-type transistor, the low potential (V_(ss))is applied to non-selected row lines, and the high potential (V_(dd)) isapplied to a selected row line R (for example, an i-th row line).

Further, similar to the first exemplary embodiment, the column selectingelement T3 and the row selecting element T4 are selection elements toselect any capacitance detecting element from the capacitance detectingelements 1 arranged in a matrix while preventing informationinterference between rows and between columns, which are dispensablecomponents. According to such a structure, by the same operation as thatin the first exemplary embodiment, a column line and a row line areselected from the column lines C and the row lines R, respectively,which results in the selection of any one from the M×N capacitancedetecting elements 1.

Since the reset element T1 and the column selecting element T3 performan on/off switching operation with the same logic, the present exemplaryembodiment is equal to the first exemplary embodiment in that the resetelement T1 and the column selecting element T3 are formed of transistorsof the same conductivity type, from the viewpoint of a simple circuitstructure. Further, the present exemplary embodiment is equal to thefirst exemplary embodiment in that a P-type transistor is used for thereset element T1, the high potential (V_(dd)) is applied to the columnlines C in a non-selected state, and the low potential (V_(ss)) isapplied thereto in a selected state, thereby obtaining the same effectsas those of the first exemplary embodiment.

Further, similar to the first exemplary embodiment, the reset element T1and the signal amplifying element T2 may be formed of thin filmsemiconductor devices having the same conductivity type. In the firstexemplary embodiment, since the source electrodes of the signalamplifying element T2 is supplied with the ground potential, which isthe potential of the power line P, a current I_(ds) flowing between thesource and drain of the signal amplifying element T2 is modulatedcorresponding to the gate potential V_(G) of the signal amplifyingelement T2. However, in the second exemplary embodiment, since thepotential V_(PL) of the power line P differs from the ground potential,a current I_(ds) flowing between the source and drain of the signalamplifying element T2 is modulated corresponding to the voltageV_(GS)=V_(G)−V_(PL) between the gate and source of signal amplifyingelement T2. During the reset period, since V_(G)=V_(PL), the voltageV_(GS) between the gate and source of signal amplifying element T2 isturned to zero. During the reading period, V_(G) is varied according tothe potential of the first electrode of the reference capacitor, thepotential of the drain of signal amplifying element and the size of thetarget capacitance. When the voltage applied to the first electrode ofreference capacitor varies ΔV_(R) in reading period with respect to thepotential of first electrode of reference capacitor in reset period, andif C_(R)>>C_(T), V_(GS) is proportional to ΔV_(R) . So V_(GS) ispositive in the reading period when ΔV_(R) is positive, and V_(GS) isnegative in the reading period when ΔV_(R) is negative. Since the firstelectrode of reference capacitor is connected to column line C, whenN-type transistors are used for the column selecting element T3 andreset element T1, a low potential (V_(ss)) is applied to non-selectedcolumn line C and a high potential (V_(dd)) is applied to selectedcolumn line C. Therefore ΔV_(R) and so V_(GS) are positive in thereading period. In this case, N-type transistor, which can amplify thesignal when V_(GS)>0, may be used for signal amplifying element T2. WhenP-type transistors are used for the column selecting element T3 andreset element T1, a high potential (V_(dd)) is applied to non-selectedcolumn line C and a low potential (V_(ss)) is applied to selected columnline C. Therefore ΔV_(R) and so V_(GS) are negative in the readingperiod. In this case, P-type transistor, which can amplify the signalwhen V_(GS)<0, may be used for signal amplifying element T2.

Next, the reset timing of the reset element T1 of an exemplary aspect ofthe present invention will be described with reference to FIG. 9. Thedescription of the reset timing in the present exemplary embodiment isalso applied to that in the first exemplary embodiment. In thecapacitance detecting device of the present exemplary embodiment, at thetiming shown in FIG. 9, a waveform is applied to allow the column lineto be selected (active) that is, the high potential (V_(dd)) is applied.For the sake of convenience, a capacitance detecting element 1 that isswitched to a selected state by allowing a row line R (for example, i)and a column line C (for example, j) to be a selected state so as todetect and read out capacitance is indicated by (i, j). For example,FIG. 6 shows a capacitance detecting element 1 (i, j+1) (the right sideof FIG. 6) and a capacitance detecting element 1 (i, j) (the left sideof FIG. 6).

As shown in FIG. 9, a column line C (for example, j+1) is in theselected state during a period {circle around (1)}, and then a columnline C (for example, j) is the selected state during a period {circlearound (2)}. As such, waveforms are sequentially supplied to therespective column lines C such that the column line C having a lowernumber is in an active state. In the present exemplary embodiment, thegate electrode of the reset element T1 in each of the capacitancedetecting elements 1 is connected to an adjacent column line that haspreviously been in the selected state. This structure makes it possibleto reset the potential of the node G immediately before the detection ofcapacitance. For example, within the period {circle around (1)}, when arow line R (i) is selected in a state in which a column line C (j+1) hasbeen in the selected state, a capacitance detecting element 1 (i, j+1)is selected, so that the period {circle around (1)} is a reading period.At this time, the capacitance detecting element 1 (i, j) is in anon-selected state. However, since the gate electrode of the resetelement T1 in the capacitance detecting element 1 (i, j) in thenon-selected state is connected to an adjacent column line C (j+1),ultimately, the period {circle around (1)} becomes a reset period by thecapacitance detecting element 1 (i, j). In the reset period {circlearound (1)}, the source electrode and drain electrode of the resetelement T1 in the capacitance detecting element 1 (i, j) areelectrically connected to each other, so that the potential V_(G) of thenode G is reset to the potential (V_(PL) in the present exemplaryembodiment, and the ground potential in the first exemplary embodiment)that is supplied to the power line P. Then, before the capacitancedetecting element 1 (i, j) enters the reading period {circle around(2)}, the column line C (j+1) returns to the non-selected state.Therefore, the reset element T1 becomes a switch-off state, and the nodeG and the power line P are disconnected from each other. When thecapacitance detecting element 1 (i, j) enters the reading period {circlearound (2)}, the column line (j) is selected, and the signalcorresponding to the electric charge stored in the capacitance detectingelectrode 41 is amplified by the signal amplifying element T2, therebymeasuring capacitance.

In the present exemplary embodiment, since a predetermined potentialV_(PL), not the ground potential, is supplied to the power line Pcontrary to the first exemplary embodiment, the analysis of voltage is alittle different from that in the first exemplary embodiment. The reasonwill be described below.

FIG. 7 is a schematic of the capacitance detecting element 1 (i, j)within the reset period {circle around (1)}, and FIG. 8 is a schematicof the capacitance detecting element 1 (i, j) within the reading period{circle around (2)}. In the reset period {circle around (1)}, thevoltage V_(G) of the node G of the capacitance detecting element 1 isreset to the supply voltage V_(PL) of the power line P. Subsequently, inthe reading period {circle around (2)}, when the voltage applied to thefirst electrode 51 of the reference capacitor varies by ΔV_(R), and thedrain voltage of the signal amplifying element T2 varies by ΔV_(T) withrespect to the voltage in reset period (FIG. 8), and when the draincapacitance of the thin film semiconductor device for signalamplification is “a·Ct” (0≦a≦1), the node voltage V_(G) is expressed bythe following Equation 10.

$\begin{matrix}\begin{matrix}{V_{G} = {V_{PL} + {{\frac{C_{R}}{C_{F} + C_{R} + C_{T}} \cdot \Delta}\; V_{R}} +}} \\{{\frac{a \cdot C_{T}}{C_{F} + C_{R} + C_{T}} \cdot \Delta}\; V_{T}}\end{matrix} & {{Equation}\mspace{14mu} 10}\end{matrix}$

On the other side, since a voltage applied to the source electrode ofthe signal amplifying element T2 is the voltage V_(PL) of the power lineP, the voltage V_(GS) between the gate and source of the signalamplifying element T2 is expressed by the following Equation 11.

$\begin{matrix}\begin{matrix}{V_{GS} = {{{\frac{C_{R}}{C_{F} + C_{R} + C_{T}} \cdot \Delta}\; V_{R}} +}} \\{{\frac{a \cdot C_{T}}{C_{F} + C_{R} + C_{T}} \cdot \Delta}\; V_{T}}\end{matrix} & {{Equation}\mspace{14mu} 11}\end{matrix}$

In the circuit structure shown in FIG. 6, since the first electrode 51of the reference capacitor 5 is connected to the column line C (j),ΔV_(R)=V_(dd). The signal amplifying element T2, the column selectingelement T3, and the row selecting element T4 are connected to each otherin series between the power line P and the output line O, and adifference in potential between the power line P and the output line Ois generally less than V_(dd) at the time of reading. Therefore,ΔV_(T=b·V) _(dd) (0≦b≦1). When this relationship is substituted intoEquation 10, the following Equation 12 is obtained.

$\begin{matrix}{V_{G} = {V_{PL} + {\frac{C_{R} + {a \cdot b \cdot C_{T}}}{C_{F} + C_{R} + C_{T}} \cdot V_{DD}}}} & {{Equation}\mspace{14mu} 12}\end{matrix}$

Further, in Equation 12, when V_(PL) is the ground potential, thefollowing Equation 13 is obtained.

$\begin{matrix}{V_{G} = {\frac{C_{R} + {a \cdot b \cdot C_{T}}}{C_{F} + C_{R} + C_{T}} \cdot V_{DD}}} & {{Equation}\mspace{14mu} 13}\end{matrix}$

When the ridge of a fingerprint touches the capacitance detectingdielectric film 42, C_(F)=C_(D). Then, when the relationship issubstituted into Equation 13, Equation 1 as in the first exemplaryembodiment is induced. When the valley of the fingerprint is locatedover the capacitance detecting dielectric film 42 with air interposedtherebetween, C_(F) =C_(D)·C_(A)/(C_(D)+C_(A)), Equation 4 in the firstexemplary embodiment is induced. That is, Equation 11 is a generalexpression relating to the detection voltage of the capacitancedetecting element 1 according to an exemplary aspect of the presentinvention, and it is confirmed that operating conditions equivalent tothe first exemplary embodiment are obtained under a specific condition.

Since the second exemplary embodiment is completely equal to the firstexemplary embodiment in circuit structure, except applying the voltageto the power line P, the effects and modifications made from the secondexemplary embodiment can be the same as those in the first exemplaryembodiment.

Further, the layer structure of the capacitance detecting elementaccording to the second exemplary embodiment is the same as that in thefirst exemplary embodiment (see FIG. 5).

As described above, since the capacitance detecting device according tothe second exemplary embodiment includes a reset element and a signalamplifying element, the second exemplary embodiment can also achieve theeffects 1) and 3) of the first exemplary embodiment.

1. A capacitance detecting device that reads surface contours of atarget by detecting capacitance which changes according to a distancefrom the target, comprising: capacitance detecting elements arranged ina matrix of M rows and N columns; and a power line to supply power tothe respective capacitance detecting elements, each of the capacitancedetecting elements having: a) a signal detecting element to storeelectric charge corresponding to the capacitance; b) a reset element toreset the electric charge stored in the signal detecting element; and c)a signal amplifying element to amplify a signal corresponding to theelectric charge stored in the signal detecting element, the signaldetecting element having a capacitance detecting electrode, the signalamplifying element being composed of a thin film semiconductor devicefor signal amplification having a source electrode, a drain electrode,and a gate electrode, the reset element being composed of a thin filmsemiconductor device for reset having a source electrode, a drainelectrode, and a gate electrode, and the gate electrode of the signalamplifying element, the capacitance detecting electrode, and the drainelectrode of the reset element being connected to each other, whereinthe gate electrode of the reset element is connected to a column lineadjacent to a column line in which the capacitance detecting elementhaving the reset element is positioned.
 2. The capacitance detectingdevice according to claim 1, when the reset element being in aswitched-on state, the gate electrode of the signal amplifying element,the capacitance detecting electrode, and the power line beingelectrically connected to each other.
 3. The capacitance detectingdevice according to claim 1, the source electrode of the reset elementbeing connected to the power line.
 4. The capacitance detecting deviceaccording to claim 1, when the capacitance detecting element is in aselected state, the source electrode of the thin film semiconductordevice for signal amplification being electrically connected to thepower line.
 5. The capacitance detecting device according to claim 1,further comprising: output lines, when the capacitance detecting elementis in the selected state, the drain electrode of the thin filmsemiconductor device for signal amplification being electricallyconnected to an output line.
 6. The capacitance detecting deviceaccording to claim 1, the signal amplifying element and the resetelement being thin film semiconductor devices having the sameconductivity type.
 7. A capacitance detecting device that reads surfacecontours of a target by detecting capacitance which changes according toa distance from the target, comprising: M row lines and N column linesarranged in a matrix of M rows and N columns; capacitance detectingelements arranged at intersections of the row lines and the columnlines; and power lines, each of the capacitance detecting elementshaving a signal detecting element, a signal amplifying element; and areset element, the signal detecting element having a capacitancedetecting electrode and a capacitance detecting dielectric film, thesignal amplifying element being composed of a thin film semiconductordevice for signal amplification having a source electrode, a drainelectrode, and a gate electrode, the reset element being composed of athin film semiconductor device for reset having a source electrode, adrain electrode, and a gate electrode, a ground potential being appliedto the power line, and the gate electrode of the signal amplifyingelement, the capacitance detecting electrode, and the drain electrode ofthe reset element being connected to each other, wherein the gateelectrode of the reset element is connected to a column line adjacent toa column line in which the capacitance detecting element having thereset element is positioned.
 8. A capacitance detecting device thatreads surface contours of a target by detecting capacitance whichchanges according to a distance from the target, comprising: capacitancedetecting elements arranged in a matrix of M rows and N columns; and aplurality of power lines to supply power to the respective capacitancedetecting elements, each of the capacitance detecting elements having:a) a signal detecting element to store electric charge corresponding tothe capacitance; b) a reset element to reset the electric charge storedin the signal detecting element; and c) a signal amplifying element toamplify a signal corresponding to the electric charge stored in thesignal detecting element, the signal detecting element including: a1) acapacitance detecting electrode; a2) a capacitance detecting dielectricfilm provided on the capacitance detecting electrode; and a3) areference capacitor, the reference capacitor having a first electrode, asecond electrode, and a dielectric film provided between the firstelectrode and the second electrode, the signal amplifying element beingcomposed of a thin film semiconductor device for signal amplificationhaving a source electrode, a drain electrode, and a gate electrode, thereset element being composed of a thin film semiconductor device forreset having a source electrode, a drain electrode, and a gateelectrode, and the gate electrode of the signal amplifying element, thecapacitance detecting electrode, the second electrode of the referencecapacitor, and the drain electrode of the reset element being connectedto each other, wherein the first electrode of the reference capacitorand a column line are connected to each other.
 9. The capacitancedetecting device according to claim 8, when the reset element is in aswitched-on state, the gate electrode of the signal amplifying element,the capacitance detecting electrode, and the second electrode of thereference capacitor being electrically connected to the power line. 10.The capacitance detecting device according to claim 8, when the resetelement is in the switched-on state, the first and second electrodes ofthe reference capacitor having the same potential.
 11. The capacitancedetecting device according to claim 8, the source electrode of the resetelement being connected to the power line.
 12. The capacitance detectingdevice according to claim 8, when the capacitance detecting element isin a selected state, the source electrode of the thin film semiconductordevice for signal amplification being electrically connected to thepower line.
 13. The capacitance detecting device according to claim 8,further comprising: output lines, when the capacitance detecting elementis in the selected state, the drain electrode of the thin filmsemiconductor device for signal amplification being electricallyconnected to an output line.
 14. The capacitance detecting deviceaccording to claim 8, the signal amplifying element and the resetelement being thin film semiconductor devices of the same conductivitytype.
 15. The capacitance detecting device according to claim 8, when acapacitance C_(R) of the reference capacitor and a transistorcapacitance C_(T) of the thin film semiconductor device for signalamplification are defined by the following expressions:C _(R)=∈₀·∈_(R) ·S _(R) /t _(R)C _(T)=∈₀·∈_(ox) ·S _(T) /t _(ox) where: ∈₀ is a dielectric constant invacuum; S_(R) (μm²) is an electrode area of the reference capacitor;t_(R) (μm) is a thickness of the reference capacitor dielectric film;∈_(R) is a relative dielectric constant of the dielectric film of thereference capacitor; S_(T) (μm²) is an area of the gate electrode of thethin film semiconductor device for signal amplification; t_(ox) (μm) isa thickness of the gate insulating film; and ∈_(ox) is a relativedielectric constant of the gate insulating film, and when a capacitanceC_(D) of the signal detecting element is defined by the followingexpression:C _(D)=∈₀·∈_(D) ·S _(D) /t _(D) where ∈₀ is a dielectric constant invacuum; S_(D) (μm²) is an area of the capacitance detecting electrode;t_(D) (μm) is a thickness of the capacitance detecting dielectric film;and ∈_(D) is a relative dielectric constant of the capacitance detectingdielectric film, the element capacitance C_(D) being sufficiently largerthan the sum of the reference capacitor capacitance C_(R) and thetransistor capacitance C_(T), that is, C_(R)+C_(T).
 16. The capacitancedetecting device according to claim 15, the capacitance detectingdielectric film being positioned on an uppermost surface of thecapacitance detecting device.
 17. The capacitance detecting deviceaccording to claim 8, the target is not brought into contact with thecapacitance detecting dielectric film, but is separated therefrom by atarget distance t_(A), and when a target capacitance C_(A) is defined bythe following equation:C _(A)=∈₀·∈_(A) ·S _(D) /t _(A) where ∈₀ is a dielectric constant invacuum; ∈_(A) is a relative dielectric constant of air; and S_(D) is anarea of the capacitance detecting electrode, the reference capacitorcapacitance C_(R) being substantially larger than the target capacitanceC_(A).
 18. The capacitance detecting device according to claim 8, thecapacitance detecting dielectric film being positioned on the uppermostsurface of the capacitance detecting device; when a reference capacitorcapacitance C_(R) and a transistor capacitance C_(T) of the thin filmsemiconductor device for signal amplification are defined by thefollowing expressions:C _(R)=∈₀·∈_(R) ·S _(R) /t _(R)C _(T)=∈₀·∈_(ox) ·S _(T) /t _(ox) where ∈₀ is a dielectric constant invacuum; S_(R) (μm²) is an electrode area of the reference capacitor;t_(R) (μm) is a thickness of the reference capacitor dielectric film;∈_(R) is a relative dielectric constant of the reference capacitordielectric film; S_(T) (μm²) is an area of the gate electrode of thethin film semiconductor device for signal amplification; t_(ox) (μm) isa thickness of the gate insulating film; and ∈₀ is a relative dielectricconstant of the gate insulating film, when an element capacitance C_(D)of the signal detecting element is defined by the following expression:C _(D)=∈₀·∈_(D) ·S _(D) /t _(D) where ∈₀ is a dielectric constant invacuum; S_(D) (μm²) is an area of the capacitance detecting electrode;t_(D) (μm) is a thickness of the capacitance detecting dielectric film;and ∈_(D) is a relative dielectric constant of the capacitance detectingdielectric film, the element capacitance C_(D) is sufficiently largerthan the sum of the reference capacitor capacitance C_(R) and thetransistor capacitance C_(T), that is, C_(R)+C_(T); and when a targetcapacitance C_(A) is defined by the following expression:C _(A)=∈₀·∈_(A) ·S _(D) /t _(A) where ∈₀ is a dielectric constant invacuum; ∈_(A) is a relative dielectric constant of air; t_(A) is adistance between the target and the capacitance detecting dielectricfilm; and S_(D) is the area of the capacitance detecting electrode, thereference capacitor capacitance C_(R) being substantially larger thanthe target capacitance C_(A).
 19. A capacitance detecting device thatreads surface contours of a target by detecting capacitance whichchanges according to a distance from the target, comprising: M row linesand N column lines arranged in a matrix of M rows and N columns;capacitance detecting elements arranged at intersections of the rowlines and the column lines; and a power line, each of the capacitancedetecting elements having: a signal detecting element; a signalamplifying element; and a reset element, the signal detecting elementhaving a capacitance detecting electrode, a capacitance detectingdielectric film, and a reference capacitor, the reference capacitorhaving a first electrode, a dielectric film, and a second electrode, thesignal amplifying element being composed of a thin film semiconductordevice for signal amplification having a source electrode, a drainelectrode, and a gate electrode, the reset element being composed of athin film semiconductor device for reset having a source electrode, adrain electrode, and a gate electrode, a ground potential being appliedto the power lines, and the gate electrode of the signal amplifyingelement, the capacitance detecting electrode, the second electrode ofthe reference capacitor, and the drain electrode of the reset elementbeing connected to each other, wherein the first electrode of thereference capacitor and a column line are connected to each other.
 20. Acapacitance detecting device that reads surface contours of a target bydetecting capacitance which changes according to a distance from thetarget, comprising: capacitance detecting elements arranged in a matrixof M rows and N columns; and a plurality of power lines to supply powerto the respective capacitance detecting elements, each of thecapacitance detecting elements having: a) a signal detecting element tostore electric charge corresponding to the capacitance; b) a resetelement to reset the electric charge stored in the signal detectingelement; and c) a signal amplifying element to amplify a signalcorresponding to the electric charge stored in the signal detectingelement, the signal detecting element including: a1) a capacitancedetecting electrode; a2) a capacitance detecting dielectric filmprovided on the capacitance detecting electrode; and a3) a referencecapacitor, the reference capacitor having a first electrode, a secondelectrode, and a dielectric film provided between the first electrodeand the second electrode, the signal amplifying element being composedof a thin film semiconductor device for signal amplification having asource electrode, a drain electrode, and a gate electrode, the resetelement being composed of a thin film semiconductor device for resethaving a source electrode, a drain electrode, and a gate electrode, andthe gate electrode of the signal amplifying element, the capacitancedetecting electrode, the second electrode of the reference capacitor,and the drain electrode of the reset element being connected to eachother, wherein the gate electrode of the reset element is connected to acolumn line adjacent to the column line in which the capacitancedetecting element including the reset element is positioned.
 21. Acapacitance detecting device that reads surface contours of a target bydetecting capacitance which changes according to a distance from thetarget, comprising: M row lines and N column lines arranged in a matrixof M rows and N columns; capacitance detecting elements arranged atintersections of the row lines and the column lines; and a power line,each of the capacitance detecting elements having: a signal detectingelement; a signal amplifying element; and a reset element, the signaldetecting element having a capacitance detecting electrode, acapacitance detecting dielectric film, and a reference capacitor, thereference capacitor having a first electrode, a dielectric film, and asecond electrode, the signal amplifying element being composed of a thinfilm semiconductor device for signal amplification having a sourceelectrode, a drain electrode, and a gate electrode, the reset elementbeing composed of a thin film semiconductor device for reset having asource electrode, a drain electrode, and a gate electrode, a groundpotential being applied to the power lines, and the gate electrode ofthe signal amplifying element, the capacitance detecting electrode, thesecond electrode of the reference capacitor, and the drain electrode ofthe reset element being connected to each other, wherein the gateelectrode of the reset element being connected to a column line adjacentto the column line in which the capacitance detecting element includingthe reset element is positioned.